European Patent 0 694 792 discloses an apparatus for detecting the position of a movable object, which has a sound signal conductor extending along a travel path and having a predetermined, uniform speed of sound propagation, as well as a signal injector, which is connected to a signal generator and is located on the movable object, for injecting a sound signal into the sound signal conductor. Arranged in this case at both ends of the sound signal conductor are signal extractors which are respectively connected to a counter, the two counters being clocked via a clock generator and connected to a subtractor for the output signals of the two counters. The output signal of the subtractor can be processed by an evaluation unit, as a measure of the delay time difference of the injected sound signal from the injecting point to the signal extractors, to form a signal which is representative of the instantaneous position of the movable object on the travel path, the signal injector operating with a signal spacing time which is greater than the sound delay time from one end of the travel path to the other.
The measured delay times from the sound injector to the signal extractors are assigned a positional value by means of the evaluation unit and a computational algorithm implemented in it. Specifically in the design of lifts, the evaluation unit is calibrated by assigning the flush point of each floor the result of the computational algorithm at this point and associating the lift position with this numerical value. Because of temperature-induced changes in buildings and/or changes in the length of the sound signal conductor, or owing to a change in the speed of sound in the sound signal conductor as a result of effects of temperature and diffusion, however, there is a shift in the assignment between the flush point and the numerical value assigned by the calibration.